Telescopic amplifier with improved common mode settling

ABSTRACT

Telescopic amplifier circuits are disclosed. In an embodiment, a telescopic amplifier includes an input stage for receiving differential input signals, an output stage for outputting differential output signals at the drains of a first output transistor and a second output transistor, a tail current transistor coupled to sources of a first input transistor and a second input transistor, a common mode feedback circuit coupled to the differential output signals and outputting a common mode output signal, and a circuit element coupled between the common mode output signal and a gate of the tail current transistor. In an embodiment the circuit element is a resistor. In another embodiment the circuit element is a source follower transistor. In additional embodiments a phase margin of the common mode feedback open loop gain of the amplifier is determined by the value of the resistor. Additional embodiments are disclosed.

RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/258,237, filed Sep. 7, 2016, which application is acontinuation of U.S. patent application Ser. No. 14/470,682, filed Aug.27, 2014, and claims priority to India Provisional Patent ApplicationNo. 3826/CHE/2013, filed Aug. 28, 2013, all of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The embodiments relate generally to the use of telescopic amplifiers.Applications of telescopic amplifiers include, for example, in analog todigital conversion using pipelined topologies with residue amplifiers,although the telescopic amplifiers are also used in additionalapplications. The embodiments advantageously provide improved commonmode settling performance in telescopic amplifiers without substantiallyincreasing circuit complexity, and without increasing power consumption.

BACKGROUND

Recent improvements for amplifiers used in analog signal applicationssuch as analog to digital conversion include the use of telescopicamplifiers. Telescopic amplifiers provide relatively high frequencyresponse with relatively low power. These telescopic amplifiers aretherefore attractive for a variety of applications, particularly andincreasingly for portable devices where low power is an importantrequirement. In an application, within a pipelined analog to digitalconverter (“ADC”) circuit topology, telescopic amplifiers areincreasingly used as the residue amplifier. In the pipelined ADC, foreach of a plurality of stages, the analog input signal is converted toone or more digital bits that approximate a magnitude of the analogsignal, e.g. the input signal is quantized. The digital bits of theoutput are then converted back to an analog signal using a digital toanalog converter (DAC), and the converted quantized signal, now ananalog voltage corresponding to the quantized value, is subtracted fromthe input signal. These functions are commonly performed using aswitched capacitor circuit known as a “MDAC”, or multiplying digital toanalog converter. The residue, which is the difference between the inputsignal and the analog to digital converted version of the quantizeddigital output signal, is then amplified in a residue amplifier. Theamplified residue signal is used as the input signal for the next stageof the pipelined ADC converter. In this manner the input analog signalis successively converted to a multiple bit digital representation inpipelined ADC stages. While other amplifiers can be used in thepipelined ADC, telescopic amplifiers are increasingly used as theresidue amplifier. Additional discussion of telescopic amplifiers may befound, for example, in U.S. Pat. No. 6,529,070, entitled “Low VoltageBroadband Telescopic Amplifier”, which is co-owned with the presentapplication, and which is hereby incorporated in its entirety herein byreference.

FIG. 1 depicts, in an example simplified circuit diagram, a telescopicoperational amplifier circuit 10 of the prior art. In FIG. 1, circuit 10has a terminal INP for receiving a positive differential signal arrangedwith a terminal INM for receiving a complementary differential inputsignal. An input stage is formed by transistor MNINP, with a gatecoupled to the positive input terminal INP, and transistor MNINM havinga gate coupled to the complementary input terminal INM. The input stagetransistors MNINP and MNINM have their respective source regions coupledtogether to form a common source terminal. The common source from thetwo input transistors MNINM and MNINP is coupled to the drain of thetail current transistor, MNTAIL.

A cascode output stage is formed by transistors MNCASP and MNCASM. Eachof these output stage transistors has a gate coupled to the cascode biasvoltage CAS_BIAS. The current sources IP and IM supply bias current tothe drain terminals of the transistors MNCASP and MNCASM. These currentsources IP=IM=I, where I is the bias current of the circuit stage. Eachof the output stage transistors MNCASP and MNCASM has a source that iscoupled to the drain of the respective one of the input stagetransistors MNINP and MNINM.

The circuit 10 has differential output terminals OUTP and OUTM fortransmitting positive and complementary differential output signals tothe next stage of the circuit. The output terminals OUTP and OUTM arecoupled to the drains of MNCASP and MNCASM, the differential outputtransistors. In order to illustrate the operation of the circuit 10, thesampling portion of the next stage circuit 20 is depicted. A switchedcapacitor circuit, the sample stage circuit 20 includes the clockedswitches implemented by transistors MNSWIP and MNSWIM. When the clocksignal CLK is true, or at a high voltage, the gates of these switchesare coupled to the CLK signal, and the transistors MNSWP and MNSWMcouple the output terminals OUTP and OUTM to the sample capacitors CPand CM in the switched capacitor sample circuit 20. The sampled valuesare then available for a later transfer into the next stage circuit (notshown).

In FIG. 1, a common mode feedback circuit 30 is depicted. This commonmode feedback circuit 30 is another switched capacitor circuit formed oftransistors MN1, MN3, MN5, and MN2, MN4, MN6, and capacitors CRFP, CRFM,CCMFBP, CCMFBN. The inputs are common mode reference signals REFCM and abias signal BIAS. The clock signal CLK and the inverted clock signalCLK_BAR are non-overlapping, complementary clocking signals. When theCLK_BAR signal is high, or true, the transistors MN2, MN4 and MN6 act asclosed switches and the capacitors CRFP and CRFM have the voltageREFCM-BIAS across them. When the CLK signal is true, or high, the commonmode feedback capacitors CCMFBP and CCMFBN are shorted to the capacitorsCRFP and CRFM, which store the common mode reference voltages. Inputterminal REFCM fixes the output common mode of the amplifier to a commonmode reference input voltage. The differential output terminals OUTP andOUTM of the telescopic amplifier are coupled to the capacitors CCMFBPand CCMFBM to form the common mode feedback path of the telescopicamplifier 10. The common mode feedback circuit has an output at nodeCMOUT.

The gate of the tail current transistor MNTAIL is coupled to the commonmode feedback circuit at node CMOUT. In this manner a common modesettling current (shown as Icm in the figure) flows through the tailtransistor MNTAIL. The common mode settling current Icm should beequally shared between the two branches of the differential circuit,shown as currents Icm/2 in FIG. 1. Differential settling current Idiffis also shown. Note that in the circuit diagram of FIG. 1, the inputstage and output stage transistors MNINP, MNINM, MNCASP, MNCASM, areeach illustrated as formed using N type MOSFET transistors, and the tailtransistor MNTAIL is also shown as formed using an N-type MOSFETtransistor. However, one skilled in the art will recognize that othertransistor types including P-type MOSFET transistors could be usedinstead of the N-type MOSFET transistors shown in this illustrativeexample, the substitution could be made to replace the N type MOSFETtransistors with P type MOSFET transistors, and in other respects, thecircuit topology and functions of the telescopic amplifier circuit 10would remain the same.

The telescopic amplifier circuit 10 of FIG. 1 may be used with apre-amplifier (not shown for simplicity) in a residue amplifier for anADC circuit to provide relatively high gain bandwidth at relatively lowpower. However, the common mode settling characteristics of the priorart telescopic amplifier 10 shown in FIG. 1 are poor. The common modefeedback loop bandwidth is low when compared to the differentialbandwidth; typically it is ½ to ⅓rd of the differential bandwidth.

As can be seen in FIG. 1, the next stage sampling circuit 20 willreceive the common mode and differential current when CLK is high. Thetransistors MNSWP and MNSWP in sampling circuit 20, also shown here asN-type MOSFET transistors, have resistances which, ideally, areperfectly matched. However in an actual physical circuit the resistancesof these two transistors will not match perfectly due to process,temperature and voltage variations (PVT). Due to the resistance mismatchof these sampling transistors, poor common mode current settling maylead to an error in the sampling circuit in the next stage. That is, adifferential current error may occur due to the presence of a commonmode settling current.

The common mode feedback loop of the circuit of FIG. 1 is effectively asingle pole system. Using a circuit analysis that neglects differentialmode signals and that is for the common mode signals only, the commonmode feedback loop gain expression for the structure in FIG. 1 is:

LPG=(β*Gmntail/(2C _(L) s+gd))  (Equation 1)

-   -   Where: β=C_(CM)/C_(CM)+C_(TAIL), and gd=the output conductance        of one arm of the telescopic structure, which is very small, and        Gmntail is the transconductance of the tail transistor MNTAIL.

From Equation 1, it can be seen that the transfer function for the loopgain, for common mode, is a single pole system with a pole located at:

P1=gd/C _(L)(radians/second).  (Equation 2)

P1 is the single, dominant pole in the common mode feedback transferfunction. In this analysis, the cascade pole and the pole due to theinput transistors were neglected, as these poles will be fractions off_(T) of the corresponding transistors, and will be located far from thecommon mode feedback unity gain bandwidth, ω_(ugh).

The capacitance C_(CM) can be determined as:

C _(CM)=(CCMFBP+CCMFBM+CRFP+CRFM), where CRFP=CRFM andCCMFBP=CCMFBM.  (Equation 3).

The tail capacitance C_(TAIL) can be determined as:

C _(TAIL) =CROUTP+C _(GDSTAIL) +C _(GDMILLER),  (Equation 4)

where CROUTP is a parasitic routing capacitance (as indicated by thedashed lines used to represent it in FIG. 1), and C_(GDSTAIL) andC_(GDMILLER) are the gate to source capacitance and gate to drain(including Miller effect) capacitances of the tail transistor MNTAIL inFIG. 1.

The load capacitance CL can be determined as:

CL=COUTP+CP,COUTM+CM,  (Equation 5)

where: COUTP=COUTM are the parasitic routing capacitances at the outputof the telescopic amplifier, (as shown by the dashed lines used to drawthese capacitors in FIG. 1) and the capacitors CM and CP, as shown inFIG. 1, are part of the sampling circuit 20 that is for the next stage.The sampling time constant is small compared to the time 1/ωugb.

Taking these factors into account, then, the unity gain bandwidthω_(ugb) for the common mode feedback loop can be determined as:

ω_(ugb) =βGmntail/2C _(L).  (Equation 6)

As discussed above, the sampling transistors MNSWIP and MNSWIM of thenext stage sampling circuit 20 in FIG. 1 may have resistances that arenot equal. The resistances may be mismatched due to process variations,temperature dependence variations, and/or a variation in voltage swing.If the common mode settling for the circuit 10 is poor, then a currentdue to common mode settling may produce a differential voltage acrossthese sampling switches and generate a sampling error in the samplingcapacitors CP and CM. This error may not be acceptable in a particularcircuit, in the high resolution residue stage for a pipelined ADCcircuit, for example. The common mode settling current may appear as adifferential current to the sampling stage, causing error.

One known approach to this problem is to try to precisely match thesampling transistor devices, MNSWIP and MNSWIM, to reduce the resistancemismatch, by tightly controlling the process, voltage and temperature(PVT) corner. This is very difficult to do in advanced semiconductorprocesses, and can reduce device yield, increasing the per device costs.Another known approach is to try and reduce the common mode current byimproving the common mode feedback settling. The unity gain bandwidthω_(ugb) could be increased but at the cost of extra power in thetelescopic structure. However, increasingly the applications for thetelescopic amplifier are for portable devices, which are often batterypowered devices, thus this increase in power consumption is alsoundesirable.

Improvements in the common mode settling characteristics for telescopicamplifiers are therefore needed to address the deficiencies and thedisadvantages of the known prior approaches. Solutions are needed thatdo not require additional power, and which do not negatively impact thenoise performance and the differential settling performance of thetelescopic amplifier circuits.

SUMMARY

The embodiments provide telescopic amplifier circuits with improvedcommon mode settling characteristics. In an embodiment, a telescopicamplifier includes an input stage for receiving differential inputsignals, an output stage for outputting differential output signals atthe drains of a first output transistor and a second output transistor,a tail current transistor coupled to sources of a first input transistorand a second input transistor, a common mode feedback circuit coupled tothe differential output signals and outputting a common mode outputsignal, and a circuit element coupled between the common mode outputsignal and a gate of the tail current transistor. In an embodiment thecircuit element is a resistor. In another embodiment the circuit elementis a source follower transistor. In additional embodiments a phasemargin of the common mode feedback open loop gain of the amplifier isdetermined by the value of the resistor.

In the embodiments, a telescopic amplifier further includes an inputstage comprising a first input transistor having a gate terminal coupledto a positive input terminal, and a second input transistor having agate terminal coupled to a complementary input terminal; an output stagecomprising a first output transistor having a source coupled to a drainof the first input transistor and having a first current source coupledto a drain of the first output transistor, and a second outputtransistor having a source coupled to a drain of the second inputtransistor, and having a second current source coupled to a drain of thesecond output transistor, a first output terminal coupled to the drainof the first output transistor, and a second output terminal coupled tothe drain of the second output transistor, the first and second outputtransistor each having gates coupled to a bias voltage terminal; a tailcurrent transistor having a drain coupled to a common source nodecoupled to the source of each of the first and second input transistors,having a source coupled to a ground potential, and having a gateterminal coupled to a tail gate node; a common mode feedback circuithaving a first feedback input coupled to the first output terminal, anda second feedback input coupled to the second output terminal, andhaving a common mode reference signal input; and having a common modeoutput; and a resistor coupled between the common mode output and thetail gate node.

In further embodiments, the telescopic amplifier includes a switchedcapacitor sampling circuit coupled to the first output terminal and tothe second output terminal, and having a first sampling transistor and afirst positive output capacitor coupled to sample the voltage at thefirst output terminal, and having a second sampling transistor and afirst complementary output capacitor coupled to sample the voltage atthe second output terminal, responsive to a clock signal coupled to thegates of the first sampling transistor and the second samplingtransistor.

In still another embodiment, in the telescopic amplifier, the commonmode feedback circuit further comprises a switched capacitor circuit. Inadditional embodiments the common mode feedback circuit furthercomprises a first transistor coupled between the first output terminaland a first plate of a first sample hold capacitor; a second transistorcoupled between a reference common mode input and the first plate of thefirst sample hold capacitor; a third transistor coupled between ancommon mode output signal and a bias node that is coupled to a secondplate of the first sample and hold capacitor; a fourth transistorcoupled between the bias node and a bias voltage input terminal; a fifthtransistor coupled between the second output terminal and a first plateof a second sample hold capacitor, the second sample hold capacitorhaving a second plate coupled to the bias node; a sixth transistorcoupled between the reference common mode input and the first plate ofthe second sample hold capacitor; a first common mode feedback capacitorhaving a first plate coupled to the first output terminal, and a secondplate coupled to the common mode output; a second common mode feedbackcapacitor having a first plate coupled to the second output terminal anda second plate coupled to the common mode output; the second, fourth andsixth transistors each having a gate coupled to an inverted clocksignal, and the first, third and fifth transistors each having a gatecoupled to a clock signal, the clock signal being non-overlapping withthe inverted clock signal.

In still another embodiment, in the telescopic amplifier describedabove, when the inverted clock signal is active, the voltage across thefirst sample hold capacitor is a reference common mode voltage receivedat the reference common mode input minus a bias voltage received at thebias voltage input, and the voltage across the second sample holdcapacitor is the reference common mode voltage received at the referencecommon mode input minus the bias voltage received at the bias voltageinput. In another embodiment, in the telescopic amplifier describedabove, whereby when the clock signal is active, the first common modefeedback capacitor is shorted to the first sample hold capacitor, andthe second common mode feedback capacitor is shorted to the secondsample hold capacitor.

In a further embodiment, in the telescopic amplifiers described above, acommon mode feedback open loop gain transfer function of the telescopicamplifier has a dominant pole due to a load capacitance comprising a sumof the first positive output capacitor and a routing capacitance at thefirst output terminal, and the common mode feedback open loop gaintransfer function of the telescopic amplifier further has a non-dominantpole due to the resistor.

In still another embodiment of the telescopic amplifier, thenon-dominant pole is located at a frequency P2 that is approximatelyequal to a quantity (C_(CM)+C_(TAIL)/RpoleC_(CM)C_(TAIL)), where acapacitance C_(CM) is a sum of the first and second common mode feedbackcapacitors and the first and the second sample and hold capacitors, acapacitance C_(TAIL) is a sum of the gate to source capacitance of thetail transistor plus the gate to drain capacitance of the tailtransistor plus a parasitic routing capacitance at the gate of the tailtransistor, and a resistance Rpole is a value of the resistor.

In another embodiment, a telescopic amplifier includes a differentialinput stage for receiving a positive input signal and a complementaryinput signal comprising a first input transistor having a gate terminalcoupled to a positive input terminal, and a second input transistorhaving a gate terminal coupled to a complementary input terminal, thefirst input transistor having a drain and the second input transistorhaving a drain; a differential output stage for outputting a positiveoutput signal and a complementary output signal comprising a firstoutput transistor having a source coupled to the drain of the firstinput transistor and having a first current source coupled to a drain ofthe first output transistor, and a second output transistor having asource coupled to the drain of the second input transistor and having asecond current source coupled to a drain of the second outputtransistor, and a first output terminal coupled to the drain of thefirst output transistor, and a second output terminal coupled to thedrain of the second output transistor; a tail current transistor havinga drain coupled to a common source node coupled to a source of each ofthe first and the second input transistors, and having a source coupledto a ground potential, and having a gate terminal coupled to a tail gatenode; a common mode feedback circuit having a first feedback inputcoupled to the first output terminal, and a second feedback inputcoupled to the second output terminal, and having a common modereference signal input; and having a common mode output; and a circuitelement coupled between the common mode output and the tail gate node;whereby a non-dominant pole is formed in a common mode feedback openloop gain transfer function of the telescopic amplifier, due to thecircuit element.

In another embodiment, a pipelined ADC converter includes an inputterminal for receiving an analog input voltage; an N-bit ADC coupled tothe input terminal for outputting a quantized digital signal; an N-bitDAC coupled to the quantized digital signal and outputting an analogvoltage corresponding to the quantized digital signal; a sample and holdcircuit coupled to sample and hold an analog input voltage received atthe input terminal; a summer circuit coupled to the sample and holdcircuit and to the analog voltage corresponding to the quantized digitalsignal, outputting a difference voltage that is a residue voltageobtained from subtracting the analog voltage corresponding to thequantized digital signal from the sample and hold analog voltage; and atelescopic amplifier coupled to the summer circuit to amplify theresidue voltage, further comprising: a differential input stage forreceiving a positive input signal and a complementary input signalcomprising a first input transistor having a gate terminal coupled to apositive input terminal, and a second input transistor having a gateterminal coupled to a complementary input terminal, the first inputtransistor having a drain and the second input transistor having adrain; a differential output stage for outputting a positive outputsignal and a complementary output signal comprising a first outputtransistor having a source coupled to the drain of the first inputtransistor and having a first current source coupled to a drain of thefirst output transistor, and a second output transistor having a sourcecoupled to the drain of the second input transistor and having a secondcurrent source coupled to a drain of the second output transistor, and afirst output terminal coupled to the drain of the first outputtransistor, and a second output terminal coupled to the drain of thesecond output transistor; a tail current transistor having a draincoupled to a common source node coupled to a source of each of the firstand the second input transistors, and having a source coupled to aground potential, and having a gate terminal coupled to a tail gatenode; a common mode feedback circuit having a first feedback inputcoupled to the first output terminal, and a second feedback inputcoupled to the second output terminal, and having a common modereference signal input; and having a common mode output; and a circuitelement coupled between the common mode output and the tail gate node,whereby a non-dominant pole is formed in a common mode feedback openloop gain transfer function of the telescopic amplifier, due to thecircuit element. In a further embodiment, in the pipelined ADC converterdescribed above, the circuit element in the telescopic amplifier furthercomprises a resistor. In a further embodiment, in the pipelined ADCconverter above, the common mode feedback open loop gain transferfunction of the telescopic amplifier has a dominant pole due to a loadcapacitance comprising the sum of an output capacitor coupled to thefirst output terminal and a routing capacitance at the first outputterminal, and the common mode feedback open loop gain transfer functionhas a non-dominant pole due to the resistor.

Previously, use of the telescopic amplifier as a residue amplifier wassometimes considered inappropriate, as the common mode settlingcharacteristics of the telescopic amplifier circuit were poor.Recognition in the embodiments of a simple solution that providesimproved common mode settling due to use of the telescopic amplifiercircuits of the embodiments surprisingly overcomes the problems anddeficiencies of the prior art circuits, without requiring additionalpower, and without degrading the differential performancecharacteristics of the telescopic amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the illustrative embodimentsdescribed herein and the advantages thereof, reference is now made tothe following descriptions taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates in a simplified circuit diagram a prior arttelescopic amplifier circuit;

FIG. 2 illustrates in a simplified circuit diagram an embodimenttelescopic amplifier;

FIG. 3 illustrates in another simplified circuit diagram an additionalembodiment telescopic amplifier;

FIG. 4 illustrates in a simplified block diagram a pipelined analog todigital converter in an example application for use with theembodiments;

FIG. 5 illustrates in a simplified circuit diagram a single stage of thepipelined analog to digital converter of FIG. 4, including a telescopicamplifier of the embodiments; and

FIG. 6 illustrates in a simplified circuit diagram a residue amplifierfor use in the single stage of the pipelined ADC converter of FIG. 5,incorporating a telescopic amplifier of the embodiments.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION

The making and using of example illustrative embodiments are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the variousembodiments, and the examples described do not limit the scope of thespecification, or the scope of the appended claims.

For example, when the term “coupled” is used herein to describe therelationships between elements, the term as used in the specificationand the appended claims is to be interpreted broadly, and is not to belimited to “connected” or “directly connected” but instead the term“coupled” may include connections made with intervening elements, andadditional elements and various connections may be used between anyelements that are “coupled”.

In the embodiments, novel solutions are provided to improving the commonmode settling performance of a telescopic amplifier. The embodimentsovercome the problems of the known prior approaches. In a firstembodiment, it is recognized that in the prior art circuits, the commonmode feedback open loop gain has a single pole. However, it iscomprehended in the embodiments that for a given common mode feedbackunity gain bandwidth, a two pole system has a faster settling time thana single pole system. Because the common mode feedback unity gainbandwidth is small compared to the other poles present in the system(because the other poles are designed for differential settling, not forcommon mode), it is surprisingly discovered that the common modesettling for the telescopic amplifier may be improved by using a twopole system, with a second non-dominant pole located away from the firstdominant pole. The second pole should be placed so that the system hasproper phase margin, that is, so the system is properly damped, but isnot overdamped, and thus has fast common mode feedback settling.

FIG. 2 illustrates in a simplified circuit diagram an example circuitembodiment 50 that provides the new second pole.

In FIG. 2, a telescopic amplifier circuit 50 is shown. In some respectsthe circuit 50 is the same as circuit 10 shown in FIG. 1. The inputsampling circuit 60 is shown coupled to the differential outputterminals OUTP and OUTM, which are taken at the drains of the cascodetransistors MNCASP and MNCASM. The differential inputs INP and INM arecoupled to the gates of input differential transistors MNINP and MNINM.The tail transistor MNTAIL is again coupled to the common sources of theinput differential transistors MNINP and MNINM. The common mode feedbackis provided by coupling the output terminals OUTP and OUTM to the inputsof switched capacitor common mode feedback circuit 70, which in thisexample is identical to the common mode feedback circuit 30 in FIG. 1.

In FIG. 2, the telescopic amplifier as shown in FIG. 1 is modified inthat an additional circuit element is placed between the output ofcommon mode feedback circuit 70, at node CMOUT, and the gate of the tailtransistor MNTAIL at node TAILGATE. In this example illustrativeembodiment, a resistance in the form of an added resistor labeled Rpoleis used for the added circuit element. The remainder of the elements oftelescopic amplifier circuit 50 shown in FIG. 2 are connected in thesame way as in telescopic amplifier circuit 10, shown in FIG. 1.

The common mode feedback open loop gain expression for the circuit 50can now be expressed as:

LPG=βGmntail/2(C _(L) s+gd)(1+(sR _(pole) C _(CM) C _(TAIL) /C _(CM) +C_(TAIL)))  (Equation 6)

The location of the new non-dominant pole is given by:

P2=C _(CM) +C _(TAIL) /RpoleC _(CM) C _(TAIL)  (Equation 7)

The location of the non-dominant pole should be selected to provideproper damping of the system. A critically damped system will have adamping factor of 1, which for a two pole transfer function will beobtained by placing the pole P2 at the frequency 4ω_(ugb). Thisfrequency for the second pole correlates to a phase margin of 76degrees, the critically damped condition for a two pole system, whichwill prevent oscillation and instability (which can occur for anunderdamped case) without unduly slowing the system (which can occur foran overdamped case). Since the embodiments provide the circuit designera variable (the value of Rpole) to control the phase margin obtained, inan embodiment the value for Rpole may be selected to obtain criticaldamping of the common mode feedback system. However, in alternativeembodiments, other similar phase margin values could be chosen byvarying the value of the added resistance Rpole, so long as the commonmode settling is improved and the system is properly damped and isstable. Accordingly, other phase margins could be selected, such as 70degrees, that have good performance, and the value for Rpole can bevaried to obtain the desired damping factor. Circuit simulations may beused to optimize the value for Rpole in a given semiconductor process.

By substitution, the value for the resistor Rpole for the criticallydamped case described above can now be determined as:

R _(pole)=(C _(L)/2β² C _(TAIL) Gmntail)  (Equation 7)

Thus the use of the telescopic amplifier circuit embodiment of FIG. 2surprisingly provides the new non-dominant pole by incorporating asimple added resistance. The common mode settling performance of thetelescopic amplifier is advantageously improved by use of theembodiments (because for a given common mode feedback unity gainbandwidth, a two pole system settles faster than a single pole system).For a small settling error, the two pole system requires only half thebandwidth of the single pole system. The advantages achieved by the useof the embodiments are surprisingly attained without added powerconsumption in the circuit. Further, the common mode settling of thetelescopic amplifier is improved with no impact on the common modefeedback bandwidth, because the added resistance does not affect thelocation of the dominant pole. The added resistance may add some noise,but because the noise is in the common mode, it is rejected in thedifferential stage. That is, any noise added by the added resistance inthe common mode feedback path is not differential noise.

FIG. 3 illustrates another embodiment telescopic amplifier 55 that alsoprovides a second, non-dominant pole that is used to improve common modesettling. In FIG. 3, a less preferred approach is used to provide thesecond pole. Instead of adding a circuit element that is a resistorbetween the common mode feedback circuit output and the gate of the tailcurrent transistor MNTAIL to create the second pole, as in FIG. 2, inthis alternative embodiment a source follower transistor, labeled MPOLE,is coupled between the output of the common mode feedback circuit 30, atmode CMOUT, and the gate of the tail transistor MNTAIL, at nodeTAIL_GATE.

While the use of the source follower transistor MPOLE in the common modefeedback path of FIG. 3 will also provide the needed second pole, andthus reduce the common mode settling time for the telescopic amplifier55 over the prior art circuits, the use of the source followertransistor MPOLE as shown in FIG. 3 will also require additional power,and requires another bias circuit Isf, and will slightly reduce thecommon mode feedback bandwidth. Thus the circuit 55 of FIG. 3illustrates an additional embodiment that is contemplated as a usefulalternative to the embodiment of FIG. 2, and which falls within thescope of the appended claims. However, the embodiment shown in FIG. 2 ispreferred over this alternative embodiment in FIG. 3, because in theembodiment of FIG. 2, no additional power is required, no added biascircuit is needed, and the overall impact on the performance of thetelescopic amplifier is less.

By adding the second pole as described in the embodiments above,improved common mode settling in telescopic amplifiers is unexpectedlyachieved with little or no additional power and with almost no reductionin the common mode feedback unity gain bandwidth. Improving the commonmode settling improves the slew rate of the telescopic amplifier circuitand the differential settling, as the biasing current settles faster inthe input transistors.

FIG. 4 illustrates in a simplified block diagram a pipelined analog todigital converter (ADC) 80 to illustrate an application where theembodiments may be advantageously used. In FIG. 4, an analog voltage isreceived at an input terminal Vin. A plurality of identical stages ofADC converters labeled Stage 1, Stage 2, etc. are arranged in successionand are coupled to one another. Each ADC stage outputs n bits ofquantized data. The number of bits n for a given stage may be 1, 2 ormore and may be as much as 5, 6 or more. That is, the number of bits ncan vary. The number of ADC stages Stage 1, Stage 2, etc. can also vary.The stages each also output a residue voltage, which is the voltage thatis input to the next successive ADC stage. The final ADC stage is only am-bit ADC converter that has no voltage residue output. The quantizedoutput is then collected, and time alignment and error correction isperformed, as shown by the block labeled “Time alignment/Errorcorrection” in FIG. 4. Finally an output that is a digitalrepresentation of the analog voltage Vin is obtained. This digitaloutput has s(n−1)+m bits.

In FIG. 4, each ADC pipelined stage, Stage 1, Stage 2 etc. can beimplemented as an identical ADC-DAC circuit. FIG. 5 illustrates theADC-DAC circuit 81 in a simplified circuit diagram. In FIG. 5, the inputvoltage Vin is an analog voltage, for example it may be a residuevoltage from the prior stage. The input voltage is received into a flashanalog to digital converter labeled “N-bit ADC” that outputs n quantizedbits. These n bits are also immediately taken into a digital to analogconverter labeled “N-bit DAC”. The output of the N-bit DAC is an analogvoltage corresponding to the voltage represented by the quantized bitsn. The input voltage Vin is also received into a sample and holdfunction labeled “S&H”. The sampled voltage is then input to a summer 83that subtracts the analog output of the N-bit DAC (analog version of thequantized voltage) from the sampled analog input voltage, forming anoutput which is the difference between the quantized voltage and theinput voltage, called the residue. In some pipelined ADC converters thatuse switched capacitor circuitry, a multiplying DAC or “MDAC” circuitperforms the sample and hold, and the N-bit DAC functions. The residuesignal is then input into amplifier 85, the residue amplifier. In atypical ADC converter, the residue amplifier 85 is a differential inputamplifier and outputs differential outputs, the amplified residuevoltage, for sampling by the next successive stage in the pipelined ADCcircuit.

In an ADC application incorporating the embodiments, the telescopicamplifiers of either FIG. 2 or FIG. 3 can be used, for example, as theresidue amplifier 85 of each of the ADC stages. Therefore additionalembodiments which are pipelined ADC circuits are also contemplated bythe inventors herein. These additional embodiments are formed by usingthe embodiment telescopic amplifiers in a pipelined ADC converter. Theseadditional embodiments are also within the scope of the appended claims.

FIG. 6 illustrates in another simplified circuit diagram an embodimentof the telescopic amplifier 90 using the added resistance RPOLE toimprove the common mode settling, configured for use as the residueamplifier of FIG. 5. In FIG. 6, telescopic amplifier 90 is a telescopicamplifier embodiment such as amplifier 50 of FIG. 2, now connected foruse as a residue amplifier 85 in the pipelined ADC stage 81 of FIG. 5,for example. The differential inputs INP and INM again are coupled tothe gates of the input stage transistors MNINP and MNINM. The cascodeoutput transistors MNCASP and MNCASM form a differential output pair andthe drain terminals are coupled to the differential outputs, now labeledVRESP and VRESM, for transmitting the residue voltage to the next stagecircuit of the pipelined ADC converter. The common mode feedback circuit91 is a switched capacitor circuit as is described above and has anoutput signal CMOUT that is coupled to the gate of tail transistorMNTAIL by the added resistor Rpole. Thus an embodiment is formed byforming a pipelined ADC converter having multiple ADC stages, eachincluding a residue amplifier that is the telescopic amplifierimplemented by using one of the embodiment telescopic amplifiers such asshown in FIG. 2, or FIG. 3, as described above, and having the improvedcommon mode settling performance.

Although the example embodiments have been described in detail, itshould be understood that various changes, substitutions and alterationscan be made herein without departing from the spirit and scope of theapplication as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure, processes, machines,manufacture, compositions of matter, means, methods or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to theembodiments and alternative embodiments. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

What is claimed is:
 1. A telescopic amplifier, comprising: an inputstage comprising a first input transistor having a gate terminal coupledto a positive input terminal, and a second input transistor having agate terminal coupled to a complementary input terminal; an output stagecomprising a first output transistor having a source coupled to a drainof the first input transistor and having a first current source coupledto a drain of the first output transistor, and a second outputtransistor having a source coupled to a drain of the second inputtransistor, and having a second current source coupled to a drain of thesecond output transistor, a first output terminal coupled to the drainof the first output transistor, and a second output terminal coupled tothe drain of the second output transistor, the first and second outputtransistor each having gates coupled to a bias voltage terminal; a tailcurrent transistor having a drain coupled to a common source nodecoupled to the source of each of the first and second input transistors,having a source coupled to a ground potential, and having a gateterminal coupled to a tail gate node; a common mode feedback circuithaving a first feedback input coupled to the first output terminal, anda second feedback input coupled to the second output terminal, andhaving a common mode reference signal input; and having a common modeoutput; and a resistor coupled between the common mode output and thetail gate node.
 2. The telescopic amplifier of claim 1, and furthercomprising a switched capacitor sampling circuit coupled to the firstoutput terminal and to the second output terminal, and having a firstsampling transistor and a first positive output capacitor coupled tosample the voltage at the first output terminal, and having a secondsampling transistor and a first complementary output capacitor coupledto sample the voltage at the second output terminal, responsive to aclock signal coupled to the gates of the first sampling transistor andthe second sampling transistor.
 3. The telescopic amplifier of claim 2,wherein the common mode feedback circuit further comprises a switchedcapacitor circuit.
 4. The telescopic amplifier of claim 3 wherein thecommon mode feedback circuit further comprises: a first transistorcoupled between the first output terminal and a first plate of a firstsample hold capacitor; a second transistor coupled between a referencecommon mode input and the first plate of the first sample holdcapacitor; a third transistor coupled between an common mode outputsignal and a bias node that is coupled to a second plate of the firstsample and hold capacitor; a fourth transistor coupled between the biasnode and a bias voltage input terminal; a fifth transistor coupledbetween the second output terminal and a first plate of a second samplehold capacitor, the second sample hold capacitor having a second platecoupled to the bias node; a sixth transistor coupled between thereference common mode input and the first plate of the second samplehold capacitor; a first common mode feedback capacitor having a firstplate coupled to the first output terminal, and a second plate coupledto the common mode output; a second common mode feedback capacitorhaving a first plate coupled to the second output terminal and a secondplate coupled to the common mode output; and the second, fourth andsixth transistors each having a gate coupled to an inverted clocksignal, and the first, third and fifth transistors each having a gatecoupled to a clock signal, the clock signal being non-overlapping withthe inverted clock signal.
 5. The telescopic amplifier of claim 4,whereby when the inverted clock signal is active, the voltage across thefirst sample hold capacitor is a reference common mode voltage receivedat the reference common mode input minus a bias voltage received at thebias voltage input, and the voltage across the second sample holdcapacitor is the reference common mode voltage received at the referencecommon mode input minus the bias voltage received at the bias voltageinput.
 6. The telescopic amplifier of claim 5, whereby when the clocksignal is active, the first common mode feedback capacitor is shorted tothe first sample hold capacitor, and the second common mode feedbackcapacitor is shorted to the second sample hold capacitor.
 7. Thetelescopic amplifier of claim 4, wherein a common mode feedback openloop gain transfer function of the telescopic amplifier has a dominantpole due to a load capacitance comprising a sum of the first positiveoutput capacitor and a routing capacitance at the first output terminal,and the common mode feedback open loop gain transfer function of thetelescopic amplifier further has a non-dominant pole due to theresistor.
 8. The telescopic amplifier of claim 7, wherein thenon-dominant pole is located at a frequency P2 that is approximatelyequal to a quantity (C_(CM)+C_(TAIL)/RpoleC_(CM)C_(TAIL)), where acapacitance C_(CM) is a sum of the first and second common mode feedbackcapacitors and the first and the second sample and hold capacitors, acapacitance C_(TAIL) is a sum of the gate to source capacitance of thetail transistor plus the gate to drain capacitance of the tailtransistor plus a parasitic routing capacitance at the gate of the tailtransistor, and a resistance Rpole is a value of the resistor.
 9. Thetelescopic amplifier of claim 8, where a value for Rpole, the resistor,is a value needed to place the non-dominant pole to provide criticaldamping of the common mode feedback open loop gain of the telescopicamplifier.
 10. The telescopic amplifier of claim 8, where a value forRpole, the resistor, is determined to be approximately equal to aquantity (C_(L)/2β²C_(TAIL)Gmntail), where a capacitance C_(TAIL) is asum of the gate to source capacitance of the tail transistor plus thegate to drain capacitance of the tail transistor plus a parasiticrouting capacitance at the gate of the tail transistor, a capacitanceC_(L) is a sum of the first output capacitor at the first outputterminal plus the parasitic routing capacitance at the first outputterminal, Gmntail is equal to a transconductance of the tail transistor,and β is a ratio (C_(CM)/C_(CM)+C_(TAIL)) where a capacitance C_(CM) isa sum of the first and second common mode feedback capacitors and thefirst and the second sample and hold capacitors.
 11. A telescopicamplifier, comprising: a differential input stage for receiving apositive input signal and a complementary input signal comprising afirst input transistor having a gate terminal coupled to a positiveinput terminal, and a second input transistor having a gate terminalcoupled to a complementary input terminal, the first input transistorhaving a drain and the second input transistor having a drain; adifferential output stage for outputting a positive output signal and acomplementary output signal comprising a first output transistor havinga source coupled to the drain of the first input transistor and having afirst current source coupled to a drain of the first output transistor,and a second output transistor having a source coupled to the drain ofthe second input transistor and having a second current source coupledto a drain of the second output transistor, and a first output terminalcoupled to the drain of the first output transistor, and a second outputterminal coupled to the drain of the second output transistor; a tailcurrent transistor having a drain coupled to a common source nodecoupled to a source of each of the first and the second inputtransistors, and having a source coupled to a ground potential, andhaving a gate terminal coupled to a tail gate node; a common modefeedback circuit having a first feedback input coupled to the firstoutput terminal, and a second feedback input coupled to the secondoutput terminal, and having a common mode reference signal input; andhaving a common mode output; and a circuit element coupled between thecommon mode output and the tail gate node, the circuit element being atleast one of a source follower transistor and a resistor; whereby anon-dominant pole is formed in a common mode feedback open loop gaintransfer function of the telescopic amplifier, due to the circuitelement.
 12. The telescopic amplifier of claim 11, wherein the circuitelement is the source follower transistor.
 13. The telescopic amplifierof claim 11, wherein the circuit element is the resistor.
 14. Thetelescopic amplifier of claim 13, wherein the common mode feedbackcircuit is a switched capacitor circuit that further comprises: a firsttransistor coupled between the first output terminal and a first plateof a first sample hold capacitor; a second transistor coupled between areference common mode input and the first plate of the first sample holdcapacitor; a third transistor coupled between an common mode output anda bias node; a fourth transistor coupled between the bias node and abias voltage input terminal, the bias node further coupled to a secondplate of the first sample hold capacitor; a fifth transistor coupledbetween the second output terminal and a first plate of a second samplehold capacitor, the second sample hold capacitor having a second platecoupled to the bias node; a sixth transistor coupled between thereference common mode input and the first plate of the second samplehold capacitor; a first common mode feedback capacitor having a firstplate coupled to the first output terminal, and a second plate coupledto the common mode output; a second common mode feedback capacitorhaving a first plate coupled to the second output terminal and a secondplate coupled to the common mode output; and the second, fourth andsixth transistors each having a gate coupled to an inverted clocksignal, and the first, third and fifth transistors having a gate coupledto a clock signal, the clock signal being non-overlapping with theinverted clock signal.
 15. The telescopic amplifier of claim 14, andfurther comprising a switched capacitor sampling circuit coupled to thefirst output terminal and to the second output terminal, and having afirst sampling transistor and a first positive output capacitor coupledto sample the voltage at the first output terminal, and having a secondsampling transistor and a first complementary output capacitor coupledto sample the voltage at the second output terminal, responsive to theclock signal.
 16. The telescopic amplifier of claim 15, where a valuefor the resistor is determined so as to place the non-dominant pole toprovide critical damping of a common mode feedback open loop gain of thetelescopic amplifier.
 17. The telescopic amplifier of claim 16, where avalue for the resistor is determined to be approximately equal to aquantity C_(L)/2β²C_(TAIL)Gmntail, where a capacitance C_(TAIL) is a sumof a gate to source capacitance of the tail transistor plus a gate todrain capacitance of the tail transistor plus a parasitic routingcapacitance at the gate of the tail transistor, a capacitance C_(L) is asum of the first positive output capacitor plus the parasitic routingcapacitance at the first output terminal, Gmntail is a transconductanceof the tail transistor, and β is a ratio C_(CM)/C_(CM)+C_(TAIL), where acapacitance C_(CM) is a sum of the first and second common mode feedbackcapacitors, and the first and the second sample and hold capacitors. 18.A pipelined ADC converter, comprising: an input terminal for receivingan analog input voltage; an N-bit ADC coupled to the input terminal foroutputting a quantized digital signal; an N-bit DAC coupled to thequantized digital signal and outputting an analog voltage correspondingto the quantized digital signal; a sample and hold circuit coupled tosample and hold an analog input voltage received at the input terminal;a summer circuit coupled to the sample and hold circuit and to theanalog voltage corresponding to the quantized digital signal, outputtinga difference voltage that is a residue voltage obtained from subtractingthe analog voltage corresponding to the quantized digital signal fromthe sample and hold analog voltage; and a telescopic amplifier coupledto the summer circuit to amplify the residue voltage, furthercomprising: a differential input stage for receiving a positive inputsignal and a complementary input signal comprising a first inputtransistor having a gate terminal coupled to a positive input terminal,and a second input transistor having a gate terminal coupled to acomplementary input terminal, the first input transistor having a drainand the second input transistor having a drain; a differential outputstage for outputting a positive output signal and a complementary outputsignal comprising a first output transistor having a source coupled tothe drain of the first input transistor and having a first currentsource coupled to a drain of the first output transistor, and a secondoutput transistor having a source coupled to the drain of the secondinput transistor and having a second current source coupled to a drainof the second output transistor, and a first output terminal coupled tothe drain of the first output transistor, and a second output terminalcoupled to the drain of the second output transistor; a tail currenttransistor having a drain coupled to a common source node coupled to asource of each of the first and the second input transistors, and havinga source coupled to a ground potential, and having a gate terminalcoupled to a tail gate node; a common mode feedback circuit having afirst feedback input coupled to the first output terminal, and a secondfeedback input coupled to the second output terminal, and having acommon mode reference signal input; and having a common mode output; anda circuit element coupled between the common mode output and the tailgate node, whereby a non-dominant pole is formed in a common modefeedback open loop gain transfer function of the telescopic amplifier,due to the circuit element.
 19. The pipelined ADC converter of claim 18,wherein the circuit element in the telescopic amplifier furthercomprises a resistor.
 20. The pipelined ADC converter of claim 19,wherein the common mode feedback open loop gain transfer function of thetelescopic amplifier has a dominant pole due to a load capacitancecomprising the sum of an output capacitor coupled to the first outputterminal and a routing capacitance at the first output terminal, and thecommon mode feedback open loop gain transfer function has a non-dominantpole due to the resistor.